Behavioral and neuroendocrine plasticity in the form of winner and loser effects

The social lives of animals are complex. Individuals living in large populations must not only navigate a variety of affi liative relationships but also a wide range of adversarial ones ( Oliveira, 2009 ). Furthermore, one’s social landscape is always in fl ux, changing in response to time of year, population density, and other stochastic environmental perturbations. Accordingly, behavioral and physiological/neural mechanisms that support social agility and fl exibility should evolve to allow individuals to fi ne-tune their behavior. One way that research has focused on this framework is by studying two related phenomena that epitomize behavioral and physiological plasticity: the winner effect and the loser effect. The winner effect is defi ned as an ability to win fi ghts following the acquisition of prior social victories, whereas the loser effect is defi ned as an increased propensity to lose fi ghts following prior social defeat. Both behavioral processes are psychological in nature, and thus each potentially can occur independently of intrinsic fi ghting ability (Hsu & Wolf, 1999). Indeed, in an important synthesis of the winner and loser effect literature, Hsu, Early and Wolf (2006 ) point out that individuals form a winner effect because they have a greater willingness to engage in a fi ght rather than by necessarily changing intrinsic ability to become faster or stronger. The same is thought to occur for the loser effect: individuals become more likely to lose because they perceive themselves as losers, as opposed to somehow becoming intrinsically slower or weaker. Winner and loser effects are found in a wide variety of taxa, including mammals ( Huhman et al., 2003 ; Oyegbile and Marler, 2005 ), reptiles ( Schuett, 1997 ), birds ( Apfelbeck, Stegherr, & Goymann, 2011 ; Drummond & Canales, 1998 ; Popp, 1988 ), fi sh ( Bakker, Feuthdebruijn, & Sevenster, 1989 ; Bakker & Sevenster, 1983 ; Beacham, 1988 ; Beaugrand, Goulet, & Payette, 1991 ; Chase, Tovey, Spangler-Martin, & Manfredonia, 2002 ), and invertebrates ( Bergman et al., 2003 ; Hoefl er, 2002 ; Whitehouse, 1997 ). Some work even suggests that humans form winner and loser effects ( Yee, Bailenson, & Duchenaut, 2009 ), while other studies have considered how these effects can ripple out and have broader effects on social behavior ( Coates, Gurnell, & Sarnyai, 2010 ). Additionally, meta-analyses of these two phenomena point out that they need not occur together – some species might show a loser effect, but not a winner effect ( Hsu et al., 2006 ;